Dalmatius spent his youth in the GallicTolosa. It is probable that his two sons, Dalmatius and Hannibalianus, were born here. During the mid-320s, Flavius Dalmatius returned to Constantinople, to the court of his half-brother, and was appointed consul and censor in 333.

In Antioch, Flavius was responsible for the security of the eastern borders of the realm. During this period, he examined the case of bishop Athanasius of Alexandria, the important opponent of the Arianism, who was accused of murder. In 334, Flavius suppressed the revolt of Calocaerus, who had proclaimed himself emperor in Cyprus. In the following year he sent some soldiers to the council of Tyros to save the life of Athanasius.

His two sons were appointed to important offices under Constantine's administration, but Flavius Dalmatius and his sons were killed in the purges that followed the Emperor's death in May 337.